Reconstructing seismic wavefields

ABSTRACT

A technique includes receiving seismic data acquired in a seismic survey in the vicinity of a reflecting interface. The survey has an associated undersampled direction. The technique includes providing second data indicative of discrete samples of incident and reflected components of a continuous seismic wavefield along the undersampled direction and relating the discrete samples to a linear combination of the continuous incident and reflected seismic wavefields using at least one linear filter. Based on the relationship, an unaliased representation of the linear combination of the continuous incident and reflected seismic wavefields is constructed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No. 12/370,684 filed Feb. 13, 2009, now U.S. Pat. No. 8,554,484 issued Oct. 8, 2013; which is incorporated herein by reference in its entirety.

BACKGROUND

The invention generally relates to reconstructing seismic wavefields.

Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations creating pressure changes and vibrations along their way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy only one type of sensors or both. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.

Some surveys are known as “marine” surveys because they are conducted in marine environments. However, “marine” surveys may be conducted not only in saltwater environments, but also in fresh and brackish waters. In one type of marine survey, called a “towed-array” survey, an array of seismic sensor-containing streamers and sources is towed behind a survey vessel.

SUMMARY

In an embodiment of the invention, a technique includes receiving seismic data acquired in a seismic survey in the vicinity of a reflecting interface. The survey has an associated undersampled direction. The technique includes providing second data indicative of discrete samples of incident and reflected components of a continuous seismic wavefield along the undersampled direction. The technique includes relating the discrete samples to a linear combination of the continuous incident and reflected seismic wavefields using at least one linear filter. Based on the relationships, an unaliased representation of the linear combination of the continuous incident and reflected seismic wavefields is constructed.

In another embodiment of the invention, a system includes an interface to receive seismic data acquired in a seismic survey in the vicinity of a reflecting interface. The survey has an associated undersampled direction. The system includes a processor to process second data, which are derived from the seismic data and are indicative of discrete samples of incident and reflected components of a continuous seismic wavefield along the undersampled direction. The discrete samples are modeled by the processor as being generated by the application of at least one linear filter to a linear combination of the continuous incident and reflected seismic wavefields. The processing of the second data includes constructing an unaliased representation of the linear combination of the continuous incident and reflected seismic wavefields based on the model

In yet another embodiment of the invention, a method includes designing a towed seismic survey. The designing includes designing the survey to enhance reconstruction of a seismic wavefield along an undersampled direction.

Advantages and other features of the invention will become apparent from the following drawing, description and claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a marine seismic acquisition system according to an embodiment of the invention.

FIG. 2 is a top view of the acquisition system of FIG. 1 according to an embodiment of the invention.

FIG. 3 is a flow diagram depicting a technique to construct an unaliased representation of a continuous upgoing vertical component of a seismic wavefield in the crossline direction according to an embodiment of the invention.

FIG. 4 is a flow diagram depicting a technique to generate data indicative of upgoing and downgoing components of a particle velocity wavefield in a crossline direction according to an embodiment of the invention.

FIG. 5 is a flow diagram depicting a technique to construct an unaliased representation of a continuous seismic wave along a crossline direction according to an embodiment of the invention.

FIG. 6 is a flow diagram depicting a technique to design and perform a towed seismic survey according to an embodiment of the invention.

FIG. 7 is a schematic diagram of a processing system according to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 depicts an embodiment 10 of a marine-based seismic data acquisition system in accordance with some embodiments of the invention. In the system 10, a survey vessel 20 tows one or more seismic streamers 30 (one exemplary streamer 30 being depicted in FIG. 1) behind the vessel 20. It is noted that the streamers 30 may be arranged in a spread in which multiple streamers 30 are towed in approximately the same plane at the same depth. As another non-limiting example, the streamers may be towed at multiple depths, such as in an over/under spread, for example.

The seismic streamers 30 may be several thousand meters long and may contain various support cables (not shown), as well as wiring and/or circuitry (not shown) that may be used to support communication along the streamers 30. In general, each streamer 30 includes a primary cable into which is mounted seismic sensors that record seismic signals. The streamers 30 contain seismic sensors 58, which may be, depending on the particular embodiment of the invention, hydrophones (as one non-limiting example) to acquire pressure data or multi-component sensors. For embodiments of the invention in which the sensors 58 are multi-component sensors (as another non-limiting example), each sensor is capable of detecting a pressure wavefield and at least one component of a particle motion that is associated with acoustic signals that are proximate to the sensor. Examples of particle motions include one or more components of a particle displacement, one or more components (inline (x), crossline (y) and vertical (z) components (see axes 59, for example)) of a particle velocity and one or more components of a particle acceleration.

Depending on the particular embodiment of the invention, the multi-component seismic sensor may include one or more hydrophones, geophones, particle displacement sensors, particle velocity sensors, accelerometers, pressure gradient sensors or combinations thereof.

For example, in accordance with some embodiments of the invention, a particular multi-component seismic sensor may include a hydrophone for measuring pressure and three orthogonally-aligned accelerometers to measure three corresponding orthogonal components of particle velocity and/or acceleration near the sensor. It is noted that the multi-component seismic sensor may be implemented as a single device (as depicted in FIG. 1) or may be implemented as a plurality of devices, depending on the particular embodiment of the invention. A particular multi-component seismic sensor may also include pressure gradient sensors, which constitute another type of particle motion sensors. Each pressure gradient sensor measures the change in the pressure wavefield at a particular point with respect to a particular direction. For example, one of the pressure gradient sensors may acquire seismic data indicative of, at a particular point, the partial derivative of the pressure wavefield with respect to the crossline direction, and another one of the pressure gradient sensors may acquire, a particular point, seismic data indicative of the partial derivative of the pressure data with respect to the inline direction.

The marine seismic data acquisition system 10 includes seismic sources 40 (two exemplary seismic sources 40 being depicted in FIG. 1), such as air guns and the like. In some embodiments of the invention, the seismic sources 40 may be coupled to, or towed by, the survey vessel 20. Alternatively, in other embodiments of the invention, the seismic sources 40 may operate independently of the survey vessel 20, in that the sources 40 may be coupled to other vessels or buoys, as just a few examples.

As the seismic streamers 30 are towed behind the survey vessel 20, acoustic signals 42 (an exemplary acoustic signal 42 being depicted in FIG. 1), often referred to as “shots,” are produced by the seismic sources 40 and are directed down through a water column 44 into strata 62 and 68 beneath a water bottom surface 24. The acoustic signals 42 are reflected from the various subterranean geological formations, such as an exemplary formation 65 that is depicted in FIG. 1.

The acoustic signals 42 that are created by the sources 40 produce corresponding subsurface-reflected acoustic signals, or pressure waves 60, which are sensed by the seismic sensors 58. It is noted that the pressure waves that are received and sensed by the seismic sensors 58 include “up going” or “incident” pressure waves that propagate to the sensors 58 after reflection in the subsurface, as well as “down going” or “reflected” pressure waves that are produced by reflections of the pressure waves 60 from an air-water boundary, or free surface 31.

The seismic sensors 58 generate signals (digital signals, for example), called “traces,” which indicate the acquired measurements of the pressure wavefield and particle motion. The traces are recorded and may be at least partially processed by a signal processing unit 23 that is deployed on the survey vessel 20, in accordance with some embodiments of the invention. For example, a particular seismic sensor 58 may provide a trace, which corresponds to a measure of a pressure wavefield by its hydrophone 55; and the sensor 58 may provide (depending on the particular embodiment of the invention) one or more traces that correspond to one or more components of particle motion.

The goal of the seismic acquisition is to build up an image of a survey area for purposes of identifying subterranean geological formations, such as the exemplary geological formation 65. Subsequent analysis of the representation may reveal probable locations of hydrocarbon deposits in subterranean geological formations. Depending on the particular embodiment of the invention, portions of the analysis of the representation may be performed on the seismic survey vessel 20, such as by the signal processing unit 23. In accordance with other embodiments of the invention, the representation may be processed by a seismic data processing system that may be, for example, located on land or on the vessel 20. Thus, many variations are possible and are within the scope of the appended claims.

A particular seismic source 40 may be formed from an array of seismic source elements (such as air guns, for example) that may be arranged in strings (gun strings, for example) of the array. Alternatively, a particular seismic source 40 may be formed from one, or, a predetermined number of air guns of an array may be formed from multiple arrays, etc. Regardless of the particular composition of the seismic sources, the sources may be fired in a particular time sequence during the survey.

Referring to FIG. 2, in accordance with embodiments of the invention, a towed marine seismic survey may have a spread 90 of streamers 30 that are spaced apart in the crossline (y) direction, which means that the seismic sensors are rather sparsely spaced apart in the crossline direction, as compared to the inline (x) spacing of the seismic sensors. As such, the seismic wavefield may be relatively densely sampled in the inline (x) direction while being sparsely sampled in the crossline direction to such a degree that the sampled seismic wavefield may be aliased in the crossline direction. In other words, the data acquired from the seismic sensors do not, in general, contain sufficient information to produce an unaliased construction (i.e., an unaliased continuous interpolation) of the seismic wavefield in the crossline direction. As a more specific example, the seismic spread 90 may contain pressure and particle velocity sensors, which have an associated sparse crossline spacing. Therefore, the data that are acquired by each type of sensors individually may not be solely sufficient to reconstruct the unaliased upgoing wavefield in the crossline direction. However, in accordance with embodiments of the invention described herein, the relationship between the upgoing and downgoing wavefields at the free surface 31 (see FIG. 1) is used to provide additional information that permits construction of an unaliased continuous representation of the upgoing pressure and/or particle velocity wavefield in the crossline direction. In the context of this application, “unaliased” means substantially free of aliasing.

The upgoing and downgoing vertical particle velocity wavefields are related through the reflection at the free surface 31. If the source depth (called “z_(s)”) is larger than the streamer depth (called “z_(r)”) or, equivalently, the direct wave has been removed from the acquired seismic data, then the relationship between the upgoing and downgoing vertical component wavefields may be described as follows:

V _(z) ⁺(ω, k _(x) , k _(y))=exp(±i2k _(z) z _(r))V _(z) ⁻(ω, k _(x) , k _(y)),   Eq. 1

where “V _(z) ⁺(ω, k_(x), k_(y))” represents the downgoing vertical component of the particle velocity; “V_(z) ⁻(ω, k_(x), k_(y))” represents the upgoing vertical component of the particle velocity; “ω” represents the frequency; “k_(x)” represents the inline wavenumber; “k_(y)” represents the crossline wavenumber; “k_(z)=√{square root over ((ω/c)²−k_(x) ²−k_(y) ²)}” represents the vertical wavenumber; and “c” represents the sound speed in the water layer. Note that the sign in the exponent on the right-hand side of equation 1 depends on the sign convention for the temporal Fourier transform and should be chosen such that for a positive extrapolation distance (twice the receiver depth “z_(r)”) multiplication of the complex exponential with the upgoing wavefield produces the expected angle-dependent delay. In the following, without loss of generality, it is assumed that the sign in the exponent has been resolved as positive.

Due to the reflection at the free surface 31, the downgoing vertical component wavefield contains different crossline spatial information than the upgoing vertical component wavefield. That is, in theory, if a certain event with a non-zero crossline incidence angle is recorded in the downgoing wavefield and the corresponding ray is traced back to the free surface reflected and extrapolated back to the streamer level, then a location is derived where the upgoing wave was not measured. However, it is typically not possible to use the above-described ray tracing. Nevertheless, the information implicit in the downgoing wavefield about the upgoing wavefield, through the reflection at the free surface may be utilized to construct the upgoing particle motion wavefield as follows.

In general, due to the relationship that is set forth in Eq. 1, one or more linear filters may be defined for purposes of relating the upgoing and downgoing samples of the vertical particle velocity wavefield to any linear combination of the up- and downgoing wavefield. More specifically, in one particular embodiment, samples of the upgoing Vz⁻(t,x,y_(i)) and downgoing Vz⁺(t, x, y_(i)) vertical particle motion wavefields (e.g., samples, which may be indirectly calculated, as further described herein) may be viewed as being obtained at the output of two linear filters that receive the upgoing vertical particle velocity wavefield. The output of these filters is sampled at one half the Nyquist rate in the crossline direction and at the full Nyquist rate in the inline direction. The two linear filters are H₁(ω, k_(x), k_(y))=1 and H₂(ω, k_(x), k_(y))=exp(i2k_(z)z_(r)) (see Eq. 1). Note that for this and other particular desired linear combinations of the continuous incident and continuous reflected wavefields, one of the filters may have a particularly simple form (unity) and therefore, by some, may be considered not to be a filter at all. As can be appreciated by one of skill in the art, for this reason, in the description herein, the terminology “one or more linear filters” or “two linear filters” is used interchangeably to describe this system.

As can be appreciated by one of skill in the art, that array measurements of two fundamental quantities in a locally (i.e., around the array) homogeneous medium, is sufficient to directionally decompose the wavefield with respect to the measurement array. In the case of an acoustic medium (i.e., a medium supporting acoustic wave propagation), and a planar horizontal array, the two fundamental quantities may be pressure and the vertical component of particle velocity. In the case of an elastic medium (i.e., a medium supporting elastic wave propagation), and a planar horizontal array, the two fundamental quantities may be the displacement and vertical traction vectors.

Furthermore, as can be appreciated by one of skill in the art, for wavefields that have been directionally decomposed and that are sufficiently sampled, the relations between the different components of particle motion and the pressure (in the acoustic case) are relatively simple, and one component may be computed from the other by spatial filtering. Thus, as an example, because the upgoing pressure can be obtained by simple spatial filtering of the upgoing vertical component of particle velocity, it is also possible to reconstruct the upgoing pressure directly from the samples of the up- and downgoing vertical particle velocity. Such variations are contemplated and within the scope of the appended claims.

Given this relationship, a construction algorithm, such as generalized sampling expansion (GSE) algorithm may be applied for purposes of determining the unaliased continuous representation of the upgoing vertical particle velocity wavefield in the crossline direction. More specifically, as described in Papoulis, A., 1977, Generalized Sampling Expansion, IEEE Trans. Cir. Syst., Vol. 24, No. 11, pp. 652-654, a band-limited function x(t) may be uniquely determined in terms of the samples g_(k)(nT) (sampled at 1/m of the Nyquist rate) of the responses g_(k)(t) of m linear systems (k=1, . . . , m) with the input x(t). Application of the GSE is described below for two approaches, according to different embodiments of the invention.

In the first approach, two linear filters H₁ and H₂ are first defined, as set forth below:

H ₁( ω, k _(x) , k _(y))=1, and   Eq. 2

H ₂( ω, k _(x) , k _(y))=exp(i2k _(z) k _(r))=(i2√{square root over (( ω/c)² − k _(x) ² −k _(y) ²)}z _(r)).   Eq. 3

As can be seen from Eqs. 2 and 3, the linear filters are functions of frequency and inline horizontal wavenumber, as well as the crossline horizontal wavenumber. The problem is solved for a fixed frequency and a fixed inline horizontal wavenumber, as denoted by the overbar above these quantities in Eqs. 2 and 3. Pursuant to the GSE, the following system may be defined:

H ₁(k _(y))Y ₁(k _(y) , y)+H ₂(k _(y))Y ₂(k _(y) , y)=1, and   Eq. 4

H ₁(k _(y) +c)Y ₁(k _(y) , y)+H ₂(k _(y) +c)Y ₂(k _(y) , y)=exp(icy), Eq. 5

where “c” represents “σ,” the Nyquist rate; “y” represents any number; and “k_(y)” represents a number between −σ and −σ+c. By using Eqs. 2 and 3 in Eqs. 4 and 5, the following relationships may be obtained:

Y ₁(k _(y) , y)+exp(i2√{square root over (( ω/c)² − k _(x) ² −k _(y) ²)}z _(r))Y ₂(k _(y) , y)=1, and   Eq. 6

Y ₁(k _(y) , y)+exp(i2√{square root over (( ω/c)² − k _(x) ²−(k _(y) +c)²)}z _(r))Y ₂(k _(y) , y)=exp(icy).   Eq. 7

Eqs. 6 and 7 define a system that may be inverted to yield two functions Y₁ and Y₂ of k_(y) and y. The determinant of the system that is defined in Eqs. 6 and 7 is the difference between two complex exponential functions, as set forth below:

det(H)=exp(i2√{square root over (( ω/c)² − k _(x) ²−(k _(y) +c)²)}z _(r))−exp(i2√{square root over (( ω/c)² − k _(x) ² −k _(y) ²)}z _(r)).   Eq. 8

Solving the system defined in Eqs. 6 and 7, yields the following relationship:

$\begin{matrix} {{\begin{pmatrix} {Y_{1}\left( {k_{y},y} \right)} \\ {Y_{2}\left( {k_{y},y} \right)} \end{pmatrix} = {\frac{1}{{H_{2}\left( {k_{y} + c} \right)} - {H_{2}\left( k_{y} \right)}}\begin{pmatrix} {H_{2}\left( {k_{y} + c} \right)} & {- {H_{2}\left( k_{y} \right)}} \\ {- 1} & 1 \end{pmatrix}\begin{pmatrix} 1 \\ {\exp \left( {\; {cy}} \right)} \end{pmatrix}}},} & {{Eq}.\mspace{14mu} 9} \end{matrix}$

which may be alternatively expressed as follows:

$\begin{matrix} {{{Y_{1}\left( {k_{y},y} \right)} = \frac{\begin{matrix} {{\exp\left( {\; 2\sqrt{\left( {\overset{\_}{\omega}/c} \right)^{2} - {\overset{\_}{k}}_{x}^{2} - \left( {k_{y} + c} \right)^{2}}z_{r}} \right)} - {{\exp \left( {\; {cy}} \right)} \cdot}} \\ {\exp\left( {\; 2\sqrt{\left( {\overset{\_}{\omega}/c} \right)^{2} - {\overset{\_}{k}}_{x}^{2} - k_{y}^{2}}z_{r}} \right)} \end{matrix}}{\begin{matrix} {{\exp\left( {\; 2\sqrt{\left( {\overset{\_}{\omega}/c} \right)^{2} - {\overset{\_}{k}}_{x}^{2} - \left( {k_{y} + c} \right)^{2}}z_{r}} \right)} -} \\ {\exp\left( {\; 2\sqrt{\left( {\overset{\_}{\omega}/c} \right)^{2} - {\overset{\_}{k}}_{x}^{2} - k_{y}^{2}}z_{r}} \right)} \end{matrix}}},{and}} & {{Eq}.\mspace{14mu} 10} \\ {{Y_{2}\left( {k_{y},y} \right)} = {\frac{\exp \left( {\; {cy}} \right)}{\begin{matrix} {{\exp\left( {\; 2\sqrt{\left( {\overset{\_}{\omega}/c} \right)^{2} - {\overset{\_}{k}}_{x}^{2} - \left( {k_{y} + c} \right)^{2}}z_{r}} \right)} -} \\ {\exp\left( {\; 2\sqrt{\left( {\overset{\_}{\omega}/c} \right)^{2} - {\overset{\_}{k}}_{x}^{2} - k_{y}^{2}}z_{r}} \right)} \end{matrix}}.}} & {{Eq}.\mspace{14mu} 11.} \end{matrix}$

Pursuant to the GSE, the functions y₁(y) and y₂(y) are determined as follows:

$\begin{matrix} {{{y_{1}(y)} = {\frac{1}{c}{\int_{- \sigma}^{{- \sigma} + c}{{Y_{1}\left( {k_{y},y} \right)}{\exp \left( {\; k_{y}y} \right)}{k_{y}}}}}},{and}} & {{Eq}.\mspace{14mu} 12} \\ {{y_{2}(y)} = {\frac{1}{c}{\int_{- \sigma}^{{- \sigma} + c}{{Y_{2}\left( {k_{y},y} \right)}{\exp \left( {\; k_{y}y} \right)}{{k_{y}}.}}}}} & {{Eq}.\mspace{14mu} 13} \end{matrix}$

Given the functions y₁(y) and y₂(y) determined using Eqs. 12 and 13, respectively, the de-aliased, continuous upgoing wavefield may be determined as follows for a fixed radial frequency and a fixed inline horizontal wavenumber:

$\begin{matrix} {{V_{Z}^{-}(y)} = {\sum\limits_{n - \infty}^{\infty}{\left\lbrack {{{V_{Z}^{-}({nY})}{y_{1}\left( {y - {nY}} \right)}} + {{V_{Z}^{+}({nY})}{Y_{2}\left( {y - {nY}} \right)}}} \right\rbrack.}}} & {{Eq}.\mspace{14mu} 14} \end{matrix}$

It is noted that in Eqs. 1-14, infinite regularly sampled data has been assumed. In practice, these infinite series may be truncated after some finite, but large, n. Thus, the various embodiments of the invention are neither limited to regular sampling nor infinite sampling, as other embodiments are contemplated and are within the scope of the appended claims.

In practice, it may not be straightforward to evaluate the integrals for y₁(y) and y₂(y), as set forth in Eqs. 12 and 13, especially since these integrals are mixed domain integrals. Therefore, in accordance with other embodiments of the invention, a different approach may be used for purposes of performing the GSE, which does not rely on the evaluation of any integrals. However, if the functions y₁(y) and y₂(y) may be obtained either analytically or by numerical quadrature, then the crossline reconstructed, dealiased upgoing wavefield may be obtained using the approach that is set forth above.

The alternative approach avoids the mixed domain integrals and may be applied in the crossline space or crossline wavenumber domain directly, as set forth in Brown, J. L., 1981, Multi-Channel Sampling of Low-Pass Signals, IEEE Trans. Cir. Syst., Vol. 28, No. 2, pp. 101-106.

In this approach, a forward system matrix A(k_(y)) may be defined as follows:

$\begin{matrix} {{A\left( k_{y} \right)} = {\begin{pmatrix} {H_{1}\left( k_{y} \right)} & {H_{2}\left( k_{y} \right)} \\ {H_{1}\left( {k_{y} + c} \right)} & {H_{2}\left( {k_{y} + c} \right)} \end{pmatrix}.}} & {{Eq}.\mspace{14mu} 15} \end{matrix}$

In the matrix A(k_(y)), H₁(k_(y)) and H₂(k_(y)) are defined in Eqs. 2 and 3 above. For crossline horizontal wavenumbers in the first subinterval (−σ, 0), the reconstruction functions P₁(k_(y)) and P₂(k_(y)) may be computed from the inverse of the matrix A(k_(y)) as follows:

P _(j)(k _(y)+(k−1)σ)=T ₁ b _(jk)(k _(y)),   Eq. 16

where “b_(jk)(k_(y))” is the jk^(th) element of the inverse of the matrix A(k_(y)); and T₁=2π/σ. The terms of the inverse matrix effectively determine P₁(k_(y)) and P₂(k_(y)) on the full interval (−σ, σ). The reconstruction functions P₁(k_(y)) and P₂(k_(y)) may then be inverse Fourier transformed to the space domain to provide the y₁(y) and y₂(y) functions, respectively. The crossline reconstructed dealiased upgoing wavefield may then be obtained using Eq. 14. Alternatively, the reconstruction may be performed directly in the crossline wavenumber domain using the reconstruction functions in Eq. 16 above.

Thus, referring to FIG. 3, in accordance with some embodiments of the invention, a technique 100 may be used for purposes of constructing an unaliased representation of a linear combination of the continuous incident and the continuous reflected seismic wavefield. Pursuant to the technique 100, data, which are indicative of discrete samples of continuous incident and continuous reflected wavefields are provided (block 104); and the samples of both the continuous incident and the continuous reflected wavefields are related (block 108) to any linear combination of the continuous incident and continuous reflected wavefields using one or more linear filters and sampling. Based on these relationships, an unaliased representation of a linear combination of the continuous incident and continuous reflected wavefields may be constructed, pursuant to block 112.

It is noted that the techniques and systems that are described herein are not limited to the crossline reconstruction of upgoing vertical particle velocity wavefields. As non-limiting examples, the techniques and systems that are described herein may equally be applied to the crossline reconstruction of an upgoing pressure wavefield, or upgoing crossline components of the particle velocity wavefield, provided that these components may be correctly decomposed into upgoing and downgoing wavefields at the sensor locations.

Additionally, although embodiments of the invention have been described herein where the upgoing and downgoing wavefields are related through reflection at the free surface 31, the techniques and systems that are described herein may likewise be applied to reflections at other surfaces, such as a reflection at a rigid surface or reflection at any other surface with a known or estimated reflection coefficient.

In accordance with some embodiments of the invention, the “samples” of the upgoing and downgoing wavefields are calculated from other acquired seismic measurements and thus, are not directly sampled. For example, the particle motion sensors of the seismic spread sample the total particle motion wavefield and do not decompose the particle motion wavefield into the upgoing and downgoing components. Therefore, in accordance with some embodiments of the invention, one or more processing steps may be applied for purposes of deriving the sampled upgoing and downgoing quantities that are then further processed to construct the unaliased continuous upgoing seismic wavefield. As a specific example, in accordance with some embodiments of the invention, techniques and systems may be applied to preprocess the seismic data similar to those described in U.S. patent application Ser. No. 12/169,260, entitled “DEGHOSTING SEISMIC DATA,” which was filed on Jul. 8, 2008 (Attorney Docket No. 14.0362-US-NP), now U.S. Pat. No. 8,116,166 issued Feb. 14, 2012, and is hereby incorporated by reference in its entirety. As described in this patent application, a technique 150 that is generally depicted in FIG. 4 may be used for purposes of processing acquired seismic data to generate data, which are indicative of the upgoing and downgoing samples of the vertical particle velocity wavefield.

Referring to FIG. 4, pursuant to the technique 150, an unaliased representation of the total (upgoing and downgoing components) of a pressure wavefield is constructed (block 154) using the pressure and the crossline pressure gradient/particle velocity data. Pursuant to block 158, the unaliased representation of the total pressure wavefield is filtered and the result is sampled at the available vertical pressure gradient/particle velocity locations, pursuant to block 158. The results from the filtering and sampling are combined (block 162) with the vertical particle velocity data to generate data, which are indicative of the upgoing and downgoing vertical particle velocity.

In addition to constructing unaliased continuous representations of upgoing wavefields, the techniques and systems that are disclosed herein may be applied to constructing wavefields other than the upgoing wavefields. More specifically, as long as the crossline aliased measured or computed up- and downgoing quantities can be related to the desired unaliased output wavefield (pressure, vertical particle velocity or otherwise) through a set of one or more linear filters, the techniques and systems that are disclosed herein may be applied. Therefore, the techniques and systems that are disclosed herein may be applied where the unaliased output quantities are pressure or the vertical or crossline component of a particle velocity, as these quantities may be related to the crossline aliased up- and downgoing wavefields through linear ghost operators, as described in U.S. patent application Ser. No. 12/131,870, entitled “JOINTLY INTERPOLATING AND DEGHOSTING SEISMIC DATA,” which was filed on Jun. 2, 2008 (Attorney Docket No. 14.0385-US-NP), now U.S. Pat. No. 7,817,495 issued Oct. 19, 2010; and is hereby incorporated by reference in its entirety.

To summarize, although specific techniques and systems are disclosed herein for constructing the unaliased continuous upgoing component of the particle velocity wavefield, the techniques and systems may be more generalized, as depicted by a technique 200 that is illustrated in FIG. 5. Referring to FIG. 5, pursuant to the technique 200, multicomponent seismic data acquired in the vicinity of a reflecting interface and aliased in at least one direction are received, pursuant to block 204. The multicomponent seismic data are processed to determine (block 208) second data, which are indicative of discrete samples of a continuous incident and reflected wavefields. The samples of both the continuous incident and the continuous reflected wavefields are related (block 212) to a linear combination of the continuous incident and continuous reflected wavefields by one or more linear filters and sampling. Based on this relationship, an unaliased representation of the linear combination of the continuous incident and the continuous reflected wavefields is constructed, pursuant to block 216.

Embodiments other than those that are specifically described herein are contemplated and are within the scope of the appended claims. For example, although a towed seismic survey has been described herein, the techniques and systems that are disclosed herein may equally be applied to any other array of seismic sensor cables. For example, in accordance with other embodiments of the invention, the techniques and systems that are disclosed herein may be applied to a seabed-based or land-based array of seismic sensor cables. Furthermore, in accordance with some embodiments of the invention, the techniques and systems that are disclosed herein may be applied to seismic data that are acquired by seismic sensors that are not part of cables (sensors that are nodes of a wireless network, for example). Thus, many variations are contemplated and are within the scope of the appended claims.

In accordance with some embodiments of the invention, the parameters of the seismic survey may be designed to enhance or facilitate the crossline construction that is described herein. More specifically, the determinant that is set forth in Eq. 8 is simply the difference between two complex exponential functions and depends on the crossline sampling distance (through c) and the streamer depth z_(r). The determinant may be calculated without performing difficult inversions and/or integrals. Therefore, the determinant may be used to evaluate the stability of the inversion for different survey depths, frequencies and crossline sampling distances before any data have been acquired. Therefore, referring to FIG. 6, in accordance with some embodiments of the invention, a technique 250 includes based at least in part on the relationships governing crossline reconstruction of a wavefield, one or more measures, which predict the expected quality of the crossline reconstruction are derived, pursuant to block 254. As examples, these measures may be based on streamer depths, survey frequency and/or crossline spacing of the seismic survey. A seismic survey may then be designed, pursuant to block 258, based at least in part on the measures; and the survey may thereafter be conducted pursuant to its design.

Referring to FIG. 7, in accordance with some embodiments of the invention, a data processing system 320 may perform at least some parts of one or more of the techniques that are disclosed herein for such purposes (as non-limiting examples) of constructing an unaliased crossline representation of a seismic wavefield; determining data indicative of samples of upgoing and downgoing wavefield components; determining a continuous crossline representation of an upgoing particle velocity wavefield; designing parameters of a seismic survey; etc. In accordance with some embodiments of the invention, the system 320 may include a processor 350, such as one or more microprocessors and/or microcontrollers. As non-limiting examples, the processor 350 may be located on a streamer 30 (see FIG. 1), located on the vessel 20 (see FIG. 1) or located at a land-based processing facility, depending on the particular embodiment of the invention.

The processor 350 may be coupled to a communication interface 360 for purposes of receiving such data as the acquired seismic data. As examples, the communication interface 360 may be a Universal Serial Bus (USB) interface, a network interface, a removable media (such as a flash card, CD-ROM, etc.) interface or a magnetic storage interface (IDE or SCSI interfaces, as examples). Thus, the communication interface 360 may take on numerous forms, depending on the particular embodiment of the invention.

In accordance with some embodiments of the invention, the communication interface 360 may be coupled to a memory 340 of the system 320 and may store, for example, various input and/or output datasets involved in the determination of the above-described upgoing and downgoing wavefield samples; construction of an unaliased continuous seismic wavefield; survey design; etc. The memory 340 may store program instructions 344, which when executed by the processor 350, may cause the processor 350 to perform various tasks of one or more of the techniques and systems that are disclosed herein, such as the techniques 100, 150, 200 and/or 250; and the system 320 may display results obtained via the technique(s)/system(s) on a display (not shown in FIG. 7) of the system 320, in accordance with some embodiments of the invention.

While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention. 

What is claimed is:
 1. A method comprising: designing a towed seismic survey having an associated undersampled direction, the designing comprising designing the survey to enhance reconstruction of a continuous seismic wavefield from data acquired in the survey along the undersampled direction.
 2. The method of claim 1, wherein the act of designing comprises: designing the survey based on a determinant calculated in the reconstruction.
 3. The method of claim 1, wherein the act of designing comprises designing a depth or crossline spacing of the survey.
 4. The method of claim 1, further comprising: conducting the designed survey. 